In 1982 the Prosthodontic Research Laboratory was established at The University of Michigan, School of Dentistry with the purpose of developing biocompatibility related investigations of materials used in restorative dentistry. One of the first projects planned by the new laboratory involved the investigation of composite resins. Clinical observations had shown that wear of some of these materials was significant, and if there is substantial loss in the occlusal vertical dimension, then disastrous consequences involving the temporomandibular joints and associated musculature can occur. A ten-year plan was developed that would bring together a team of investigators to examine wear and abrasion resistance. The Plan proposed three stages of research. The first stage would develop valid and reliable methods to quantify wear, and initiate in vivo longitudinal studies. The second stage would combine the quantitative analysis with qualitative evaluations of the surface degradation that accompanies wear. Finally, correlations between quantitative and qualitative findings would lead to investigations that would suggest new formulations for the "ideal" composite resin material. A first stage study was initiated in 1984 (#DE06720) entitled "In vivo wear of various composite resin materials". That initial study had as its purpose to develop an accurate and reliable system to measure wear, and to initiate a longitudinal in vivo investigation. This new proposal is a continuation of the initial study, and one of the stage-two studies. The specific aims of this new proposal are: 1) to further refine the measuring system developed in our initial project, 2) to continue and expand the in vivo longitudinal study and to examine the "mechanisms of wear" at the occlusal and contact free areas of the sample, 3) to compare the measures of wear with the surface characteristics of the sample as determined by light and scanning electron microscopic examinations. A project of this magnitude is both reasonable and practical, since: 1) the research team has extensive experience in dental material sciences, in vivo longitudinal studies, and SEM analysis, 2) the instrumentation and experience to measure wear has been developed during the DE#06720 project, and 3) the biostatistical support for analysis and evaluation of the collected data and the maintenance of the scientific method has been established early in our planning.